Air Cooled Heat Exchangers (ACHE) are well known in the art and are used for cooling in a variety of industries including power plants, petroleum refineries, petrochemical and chemical plants, natural gas processing plants, and other industrial facilities that implement energy intensive processes. ACHE exchangers are used typically where there is lack of water, or when water usage permits cannot be obtained, or where there is not sufficient real estate to build a tower. ACHEs lack the cooling effectiveness of “Wet Towers”.
Typically, an ACHE uses a finned-tube bundle with rectangular box headers on both ends of the tubes. Cooling air is provided by one or more large fans. Usually, the air blows upwards through a horizontal tube bundle. The fans can be either forced or induced draft, depending on whether the air is pushed or pulled through the tube bundle. The space between the fan(s) and the tube bundle is enclosed by a plenum chamber which directs the air (flow field) over the tube bundle assembly thereby providing cooling. The whole assembly is usually mounted on legs or a pipe rack. The fans are usually driven by electric induction motors through some type of speed reducer. The speed reducers are typically V-belts, HTD drives, or right-angle gears. The fan drive assembly is supported by a steel mechanical drive support system. They usually include a vibration switch on each fan to automatically shut down a fan which has become imbalanced for some reason. Airflow is very important in ACHE cooling to ensure that the air has the proper “flow field” and velocity to maximize cooling. Turbulence and “choked-flow conditions can impair cooling efficiency. Therefore, mass airflow is the key parameter to removing heat from the tube and bundle system. ACHE cooling differs from “wet” cooling (i.e. wet cooling towers) in that ACHE systems do not use water to cool the tube bundle and thus, do not benefit from the latent heat of vaporization or “evaporative cooling”.
Prior art ACHE fan drive systems use any one of a variety of fan drive components. Examples of such components include electric motors, steam turbines, gas or gasoline engines, or hydraulic motors. The most common drive device is the electric motor. Steam and gas drive systems have been used when electric power is not available. Hydraulic motors have also been used with limited success. Specifically, although hydraulic motors provide variable speed control, they have relatively low efficiencies.
Fan-tip speed should not exceed 12,000 feet per minute for mechanical reasons, and may be reduced to obtain lower noise levels. Motor and fan speed are sometimes controlled with variable frequency drives. The most commonly used speed reducer is the high-torque, positive type belt drive, which uses sprockets that mesh with the timing belt cogs. They are used with motors up to 50 or 60 horsepower, and with fans up to about 18 feet in diameter. Banded V-belts are still often used in small to medium sized fans, and gear drives are used with very large motors and fan diameters. Fan speed is set by using a proper combination of sprocket or sheave sizes with timing belts or V-belts, and by selecting a proper reduction ratio with gears. In many instances, right-angle gear boxes are used as part of the fan drive system in order to translate and magnify torque from an offset electrical motor. However, belt drives, pulleys and right-angle gear boxes have poor reliability.
The aforesaid complex, prior art mechanical drive systems require stringent maintenance practices to achieve acceptable levels of reliability. In particular, one significant problem with ACHE fan systems is the poor reliability of the belt due to belt tension. A common practice is to upgrade to “timing belts” and add a tension system. One technical paper, entitled “Application of Reliability Tools to Improve V-Belt Life on Fin Fan Cooler Units”, by Rahadian Bayu of PT. Chevron Pacific Indonesia, Riau, Indonesia, presented at the 2007 International Applied Reliability Symposium, addresses the reliability and efficiency of V-belts used in many prior art fan drive systems.
The reliability deficiencies of the belt and pulley systems and the gear reducer systems used in the ACHE fan drive systems often result in outages that are detrimental to mission critical industries such as petroleum refining, petro-chemical, power generation and other process intensive industries dependant on cooling. Furthermore, the motor systems used in the ACHE fan drive systems are complex with multiple bearings, auxiliary oil and lubrications systems, complex valve systems for control and operation, and reciprocating parts that must be replaced at regular intervals. Many petroleum refineries, power plants, petrochemical facilities, chemical plants and other industrial facilities utilizing prior art ACHE fan drive systems have reported that poor reliability of belt drive systems and right-angle drive systems has negatively affected production output. These industries have also found that service and maintenance of the belt drive and gearbox system are major expenditures in the life cycle cost, and that the prior art motors have experienced failure due to the mis-use of high pressure water spray.
The duty cycle required of an ACHE fan drive system is extreme due to intense humidity, dirt and icing conditions, wind shear forces, corrosive water treatment chemicals, and demanding mechanical drive requirements.
In an attempt to increase the efficiency of ACHE cooling systems, some end-users spray water directly on the ACHE system to provide additional cooling on process limiting, hot days. Furthermore, since fan blades can become “fouled” or dirty in regular service and lose performance, many end-users water-wash their ACHE system to maintain their cooling performance. However, directly exposing the ACHE system to high pressure water spray can lead to premature maintenance and/or failure of system components, especially since prior art drive systems are typically open thereby allowing penetration by water and other fluids.
Refining of petroleum cannot take place without cooling. Refineries process hydrocarbons at high temperatures and pressures. The loss of cooling within a refinery can lead to unstable and dangerous operating conditions requiring an immediate shut down of processing units. Cooling systems have become “mission critical assets” for petroleum refinery production. As demand for high-end products such as automotive and aviation fuel has risen and refining capacity has shrunk, the refineries have incorporated many new processes that extract hydrogen from the lower value by-products and recombined them into the higher value fuels, improving yield. Many of these processes depend on cooling to optimize the yield and quality of the product. Refining processes also incorporate many advanced processes that need reliable cooling systems to protect profitability. Cooling reliability has become mission critical to refinery profit and is affected by many factors such as environmental limitations on cooling water usage, inelastic supply chain pressures and variable refining margins. Many refineries have been adding processes that reform low grade petroleum products into higher grade and more profitable products such as aviation and automotive gasoline. These processes are highly dependent upon cooling systems to control the process temperatures and pressures that affect the product quality, process yield and safety of the process. In addition, these processes have tapped a great deal of the cooling capacity reserve leaving some refineries “cooling limited” on hot days and even bottlenecked. Most U.S. refineries operate well above 90% capacity and thus, uninterrupted refinery operation is critical to refinery profit and paying for the process upgrades implemented over the last decade. The effect of the interruption in the operation of cooling units with respect to the impact of petroleum product prices is described in the report entitled “Refinery Outages: Description and Potential Impact On Petroleum Product Prices”, March 2007, U.S. Department of Energy.
Thus, the efficiency and production rate of a process is heavily dependent upon the efficiency of the ACHE cooling fan drive system and its ability to remove heat from the system.
Therefore, in order to prevent supply interruption of the inelastic supply chain of refined petroleum products, the reliability and subsequent performance of ACHE fan drive systems must be improved and managed as a key asset to refinery production and profit. An efficient and reliable fan drive system is required to maintain a relatively high cooling efficiency and prevent interruptions in production.